1. Field of the Invention
The subject invention relates to an assembly for sensing moisture on a sheet of glass and, more particularly, to a rain sensor for detecting rain on the windshield of an automotive vehicle to turn on the wipers.
2. Description of the Prior Art
As noted in the abovementioned co-pending application, a focal plane array of photosensors configured as CCD, CMOS or other means, is capable of detecting rain or other forms of moisture on a vehicle windshield. In general, a lens or reflective optical element is used to form a real image, in the optical sense, of the objects on the focal plane array, and that array is then read by addressing each image element or pixel of the array sequentially. The analog output level of each pixel is a function of the integrated light exposure of that image element, and is normally converted into a digital value which is stored in a memory buffer. The aggregate of such sequentially addressed and stored digital pixels constitutes an image frame, which may consist of only a few thousand pixels or over a million pixels in high resolution imagers.
In many imaging applications, such as in conventional television, the stored digital data that forms the image frame is simply read out and is used to create an analog image on a display screen. Other uses for such digitally stored images include metrology and/or machine vision applications where the digitally stored image frame is subjected to image analysis. The present invention describes a method whereby stored image frames are analyzed digitally with certain algorithms, in order to recognize the presence of specific characteristic images. Such images might include water drops, rivulets, mist, or even dirt.
A method for sensing moisture on the exterior surface of a sheet of glass comprising the steps of positioning an imaging lens in spaced relationship to the interior surface of the sheet of glass, passing light waves from moisture on the exterior surface of the sheet of glass through the imaging lens and producing first and second successive images of the moisture on the glass, and successively directing the first and second images from the lens onto a focal plane detector. The method is characterized by storing the first image, storing the second image, comparing the first and second images, and providing a control signal in response to predetermined differences between the first and second images.
The assembly to perform the method includes a focal plane detector adapted for disposition in adjacent to the interior surface of the sheet of glass, and an imaging lens adapted for disposition between the focal plane detector and the interior surface of the sheet of glass for directing light waves from moisture on the exterior surface of the sheet of glass through the imaging lens to produce successive first and second images of the moisture on the focal plane detector. The assembly is characterized by a controller for storing the first image and for storing the second image and for comparing the first and second images, and for providing a control signal in response to predetermined differences between the first and second images.